The present invention relates to a photosensitive resin composition, which is suitable for forming photo-via-holes when a multilayer circuit board is formed by a build up system, to an insulating film obtainable by curing the above composition, and to the multilayer circuit board provided with the above insulating film.
Currently, printed circuit boards are significantly improved with the aim of reducing size and weight of electronic devices. Particularly, in order to respond to a high density mounting demand such as bare chips, CSP (chip size package), and BGA (ball grid array), manufacturing methods of the multilayer circuit board by build up system using lithography, wherein interlayer connection is performed by via-holes having high freedom, are widely used. And, high performance a photosensitive resin composition usable in these methods have been required.
In accordance with high density surface mounting, particularly, such as BGA and bare chip mounting which currently come to receive wide attention, the multilayer circuit board itself, i.e. a base material for the mounting, is exposed to a high temperature in the mounting processes such as a reflow process, a wire bonding process, and the like. Therefore, an insulating layer must be formed with a photosensitive resin having a high glass transition point (Tg) in order to prevent mounting members from deteriorating various characteristics such as an adhesive force and an elastic modulus under a high temperature in the mounting processes and repairing processes. As the photosensitive resin having a high glass transition point, resins containing epoxy resin as a main component, which are cured by cathion polymerization using a photo-acid generating agent, are disclosed. For instance, technology of photosensitive resin composition, of which main component is well known epoxy resin, has been disclosed in JP-B-49-17040 (1974). Technology using mainly epoxy resin and vinyl compounds have been disclosed in JP-A-54-48881 (1979), JP-A-56-55420 (1981), and JP-A-5-273753 (1993).
In accordance with manufacturing processes of the multilayer circuit board by build up system using lithography as indicated in FIG. 1, generally, a film of photosensitive material (a photosensitive resin composition) is formed onto surface of a core substrate 3, which has been provided with an inner layer conductor 1 at the surface (FIG. 1(a)); through-holes for forming via-holes reaching at the inner circuits in a lower layer through the photosensitive layer 2 are formed by exposing the photosensitive film 2 via a designated mask and developing (FIG. 1(b)); surface of the insulating layer (including inner wall of the through holes for forming the via-holes) is roughened by an oxidizing agent in order to ensure an adhesiveness with a circuit conductor (such as plated copper, and the like) which is subsequently formed (FIG. 1(c-1)); and after forming a plated resist film 6 having a designated pattern on the roughened surface, outer circuits 7 (flat circuit and via-holes circuit) are formed at the exposed portion of the roughened plane by plating (FIG. 1(d-1)).
However, a curing degree of the photosensitive resin composition differs depending on a depth from surface of the film at the exposure, because the exposure dose of ultraviolet rays differs depending on the depth by self absorption of the ultraviolet rays. Therefore, the curing degree differs significantly between the surface and the bottom of the film. Because the photosensitive resin composition has different ultraviolet ray absorption depending on the portions of the film, a significant difference in the curing degree of the upper portion and the bottom portion of the film is generated at the exposure. Accordingly, if the surface roughening operation is performed after exposure and development of the film, the progress of roughening at the bottom portion of the through holes for via-hole formation becomes larger than that of the upper portion of the film. Therefore, practically, problems such as peeling the insulating film, and expanding the inner diameter at the bottom portion 5 of the through hole for via-hole formation 4 (i.e forming a reverse tapered shape) as indicated in FIG. 1(c-2) and (d-2) are generated. The reverse tapered shape portion 5 of the through hole for via-hole formation is not desirable, because it may cause a break of via-hole circuit, a crack generation in the insulating layer, and others.
In order to prevent the above problems, and to roughen the inner wall of the through hole for via-hole formation uniformly, progressing the curing of the insulating layer before the roughening process by adding a sensitizer to the photosensitive resin composition, increasing the exposure dose, heat treating after the development, and the like can be considered. However, if doing so, the surface portion of the insulating film is cured excessively, and sufficient adhesion with the flat circuit conductor can not be obtained, because the surface is not roughened desirably in the roughening process.
Accordingly, in accordance with the prior art, roughening and adhesiveness of the insulating film could not be compatible each other. The above problem is significant with the insulating film, of which film thickness is equal to or more than 40 .mu.m, particularly, in accordance with the prior art, a via-hole of more than 25 .mu.m in diameter could not be formed in an insulating film of more than 45 .mu.m thick.